This invention relates to an ultrasonic detection apparatus, and more particularly to an ultrasonic detection apparatus which can remove the influence of quantization side lobes on a tomographic image.
Detection apparatus utilizing acoustic energy has been embodied as an ultrasonic diagnosis apparatus for forming a tomographic image of a patient body. The ultrasonic diagnosis apparatus may be classified into a mechanical scan type and an electronic scan type. In the electronic scan type apparatus, a probe is formed by an array of oscillators (transducers) arranged linearly and a sector or linear scan is effected by controlling the timing of transmission and reception of acoustic energy at respective oscillators. For instance, in an electronic linear scan type apparatus, in order to focus beams of ultrasonic energy emitted from a plurality of oscillators into a given position, delay circuits having different delay times are inserted between a driving circuit, i.e. a pulse source and the oscillators. Heretofore, the delay circuits have been composed of a lumped circuit comprising a delay line with taps. In an electronic sector scan type apparatus, there are also provided a plurality of delay circuits having different delay times in order to focus the transmitted ultrasonic beams as well as to deflect the focused beams over a given angle.
In order to improve resolution of a displayed image in the ultrasonic diagnosis apparatus, the driving pulse must have a high frequency and a wide bandwidth. However, the characteristics of the delay line such as its frequency property, group delay property and reflection property are liable to be worse in proportion to the increase of the driving pulse frequency and bandwidth. For eliminating such a drawback, the number of stages of the tapped delay line should be increased and L and C should be accurately determined. Therefore, the delay line becomes expensive exponentially.
The inventor has already proposed an ultrasonic diagnosis apparatus utilizing a surface wave delay device which can overcome the drawbacks of the usual delay line. This surface wave delay device comprises a surface wave propagating medium, at least one first electroacoustic transducer provided on a surface of the medium, and a plurality of second electroacoustic transducers provided on the same surface of the medium in such a manner that distances between the first electroacoustic transducer and respective second electroacoustic transducers are made different. By utilizing such a surface wave delay line, the above mentioned drawbacks can be removed, but there is still another problem to be solved.
In the electronic scan type apparatus, since the probe comprises a plurality of oscillators arranged in a discrete manner, the generation of quantization side lobes in addition to a main lobe is inevitable. It has been confirmed that an influence of the quantization side lobes upon a tomographic image can be removed by decreasing an output level of the quantization side lobes by a given value such as 40 dB. However, in practice, it is very difficult to decrease the output level of the quantization side lobes by the predetermined amount and therefore, it is difficult to suppress effectively the influence of the side lobes upon the tomographic image.